1. Field of the Invention
The present invention generally relates to a tunable wavelength light source, particularly, to an external-cavity tunable waveguide light source using a semiconductor laser, and more particularly, to a tunable wavelength optical source in which an optical filter using an interferometer serves as part of an external cavity to tune the oscillation wavelength of a laser beam at good reproducibility and a high resolving power, thereby improving various characteristics such as a side mode suppression ratio and a spectral line width.
2. Description of the Related Art
As a typical conventional external-cavity tunable wavelength light source using a semiconductor laser and used in the above field, a tunable waveguide light source in which a wavelength is selected by an external diffraction grating is known.
FIG. 9 shows the arrangement of a conventional external-cavity tunable waveguide light source.
A light beam emerging from an AR-coated (AR: Anti-Reflection) facet la of a semiconductor laser (LD) 1 is converted into a collimated beam by a lens 2, and the parallel beam is incident on a diffraction grating 3. The collimated beam is spectrally dispersed by the diffraction grating 3, and only a light component having a specific wavelength (to be described later) is returned to the LD 1. In this manner, a cavity is formed between a facet 1b of the LD 1 which is not AR-coated and the diffraction grating 3, thereby oscillating a laser beam having a wavelength determined by a cavity length L. An output laser beam is emitted from the facet 1b of the LD 1 which is not AR-coated.
Note that an oscillation wavelength setting unit 4 outputs a rotary drive signal to rotate the diffraction grating 3 in a direction of a two-headed arrow R using a rotary shaft 3a as a fulcrum, thereby setting an incident angle .theta. of the light beam which is incident from the lens 2 on the diffraction grating 3.
The principle of the above laser oscillation will be described below.
Light beams incident on the diffraction grating 3 are diffracted at different angles depending on the wavelengths of the light beams. More specifically, as shown in FIG. 10, when the grating constant of the diffraction grating 3 is represented by d, and an incident angle with respect to the diffraction grating 3 is represented by .theta., a wavelength .lambda. at which an exit angle .beta. is obtained satisfies the following equation: EQU m.lambda.=d(sin.theta.+sin.beta.), (1)
(where m is a diffracted order which can be set to .+-.1, .+-.2, . . . )
In the light beam incident on the diffraction grating 3, a light component having a wavelength which satisfies .theta.=.beta. in equation (1) is returned to the LD 1, thereby forming a cavity (cavity length L) between the diffraction grating 3 and the LD 1. A wavelength of a laser beam oscillated at this time, as shown in FIG. 11, is determined by the gain spectrum of the LD 1, the wavelength characteristics of a cavity loss (mainly, characteristics of the diffraction grating 3), and an external-cavity longitudinal mode determined by the phase condition of a light beam. That is, the laser beam is oscillated in an external-cavity longitudinal mode at which a value obtained by subtracting the loss from the gain becomes maximum.
This external-cavity longitudinal mode is a condition for forming a standing wave when a light beam reciprocally travels in the cavity, and is given by the following equation: EQU n.lambda.=2 L (2)
(n: a natural number, L: the above cavity length)
At this time, each external-cavity longitudinal mode interval .DELTA..lambda. is given by: EQU .DELTA..lambda.=.lambda..sup.2 /2 L (3)
Referring to FIG. 11, when the incident angle .theta. with respect to the diffraction grating 3 is changed, a wavelength at which the loss of the cavity becomes minimum changes in the direction of a two-headed arrow W indicated by a dotted line in FIG. 11. More specifically, according to this scheme, when the diffraction grating 3 is rotated, an arbitrary external-cavity longitudinal mode can be selected within a range in which the LD 1 has a large gain width.
Note that, in order to sequentially oscillate laser beams in the external-cavity longitudinal modes shown in FIG. 11, the diffraction grating 3 is rotated, and the incident angle .theta. must be set at external-cavity longitudinal mode intervals .DELTA..lambda..
At this time, the selectivity of the external-cavity longitudinal modes depends on the resolving power of the diffraction grating 3.
In the external-cavity tunable wavelength light source using such a diffraction grating, the following conditions must be satisfied to realize the tunable wavelength light source:
1 A spectral line width is decreased.
2 A side mode suppression ratio is increased, i.e., the influences of modes except for a selected external-cavity longitudinal mode are reduced.
In order to realize the condition 1, in FIG. 9, a distance between the LD 1 and the diffraction grating 3, i.e., the cavity length L, must be increased. However, in this case, the following problems (a) and (b) are posed.
(a) As is apparent from equation (3), when the cavity length L is increased, an external-cavity longitudinal mode interval .DELTA..lambda. decreases. For this reason, since the selectivity of the external-cavity longitudinal modes of the diffraction grating 3 is degraded, the reproducibility of an oscillation wavelength is degraded. In addition, since a cavity loss difference between the modes decreases, a side mode suppression ratio decreases. Therefore, the condition 2 is not satisfied.
In order to solve this, the resolving power of the diffraction grating 3 may be increased. However, the resolving power of the diffraction grating 3 is proportional to a grating constant d, and this grating constant d is minimized to a physical limit at present.
In order to solve the problem (a) and satisfy the condition 2, in FIG. 9, a method of inserting an optical filter (wavelength filter) into a cavity constituted by the LD 1 and the diffraction grating 3 to improve the selectivity of external-cavity longitudinal modes is considered.
As a scheme for realizing this method, a scheme in which a Fabry-Perot etalon (to be referred to as an etalon hereinafter) is used as an optical filter is conventionally proposed ("External-Cavity Laser Design and Wavelength Calibration" HP Journal, 1993, February pp. 20-27). However, this scheme has the following problem.
(b) Although a setting precision for the cavity length of the etalon requires a 10.sup.-12 m order, this cavity length is difficult to be controlled. In addition, in order to avoid that the reflected light beam of a light beam incident on the etalon is returned to the LD 1, the etalon must be arranged to be inclined with respect to an optical axis. For this reason, the cavity length is more difficult to be controlled.
The above explanation will be more exactly described below.
The FSR (Free Spectrum Range) of the etalon is determined by a distance l between two mirrors as follows:
FSR=C/2nl
(where C: light velocity, n: refractive index).
For example, assuming that the FSR is set to be 50 GHz, nl.perspectiveto.3 mm in a normal state, n.perspectiveto.1, in air.
A change amount .DELTA.l of l required when a transmission peak wavelength is changed by .DELTA..lambda. is expressed by EQU .DELTA.l=l.multidot..DELTA..lambda./.lambda.
In order to obtain a change represented .DELTA..lambda.=1 pm when .lambda.=1.5 .mu.m, .DELTA..perspectiveto.2.times.10.sup.-12 m must be satisfied. It is difficult to satisfy this condition by means of a mechanical device.